1. Field of the Invention
The present invention relates to a disk drive and to a spindle motor for a disk drive. More particularly, the present invention relates to a magnetic hard disk drive ("HDD") having a spindle motor having an integral flange land portion for reducing the effects of temperature-related differential expansions between the constituent elements of the spindle motor.
2. Description of the Prior Art and Related Information
A typical hard disk drive includes a head disk assembly ("HDA") and a printed circuit board assembly ("PCBA"). The HDA includes at least one magnetic disk ("disk"), a spindle motor for rotating the disk, and a head stack assembly ("HSA") that includes a read/write head for reading and writing data. The HSA is controllably positioned by a servo system in order to read or write information from or to particular tracks on the disk. The typical HSA has three primary portions: (1) an actuator assembly that moves in response to the servo control system; (2) a head gimbal assembly ("HGA") that extends from the actuator assembly and biases the head toward the disk; and (3) a flex cable assembly that provides an electrical interconnect with minimal constraint on movement.
A "rotary" or "swing-type" actuator assembly comprises a body portion that rotates on a pivot bearing cartridge between limited positions, a coil portion that extends from one side of the body portion to interact with one or more permanent magnets to form a voice coil motor, and an actuator arm that extends from an opposite side of the body portion to support the HGA.
A typical HGA includes a load beam, a gimbal attached to an end of the load beam, and a head attached to the gimbal. The load beam has a spring function that provides a "gram load" biasing force and a hinge function that permits the head to follow the surface contour of the spinning disk. The load beam has an actuator end that connects to the actuator arm and a gimbal end that connects to the gimbal that carries the head and transmits the gram load biasing force to the head to "load" the head against the disk. A rapidly spinning disk develops a laminar air flow above its surface that lifts the head away from the disk in opposition to the gram load biasing force. The head is said to be "flying" over the disk when in this state.
Within the HDA, the spindle motor rotates the disk or disks, which are the media to and from which the data signals are transmitted via the head(s). The transfer rate of the data signals is a function of rotational speed of the spindle motor; the faster the rotational speed, the higher the transfer rate. The density of the information stored on a disk is a function, among other factors, of the number of Tracks Per Inch (TPI) on the disk. The higher the TPI, the higher the storage density, all other factors being equal. To increase disk storage capacity and transfer rate, disk drive designers have found it expedient to increase both TPI and disk rotational speeds. However, increased TPI and disk rotational speeds render the correct and dependable positioning of the heads over the disk an increasingly delicate matter. Head positioning errors (as measured by a signal called the Position Error Signal (PES) or some equivalent signal) are a primary concern in the design and manufacture of disk drives.
Moreover, the frequency and severity of these head positioning errors are exacerbated by fluctuations in temperature. Indeed, whereas the hub flange supporting the disks on the spindle motor and the disks themselves may be formed of or include aluminum, the yoke supporting the spindle motor magnet is typically made of a magnetic material, such as steel. Dissimilar materials typically have different thermal coefficients of expansion. As the temperature of the drive changes, these different thermal expansion coefficients cause the hub, the disks (and the disk spacers) and the yoke to expand and contract at different rates, causing the disks to undergo undesired radial shifts, resulting in an increased incidence of head positioning errors.
The disks of magnetic disk drives, despite the most exacting manufacturing specifications, typically vary in their "roundness" and in their thickness. When such disks are secured to the spindle motor of the drive, these slight variations in roundness and thickness, for example, cause the disk to deform somewhat, assuming a smoothly varying topography that is sometimes likened to that of a potato chip. Radial shifts, caused by temperature cycling, can change the shape of this potato chip pattern. Because of such radial shifts, the read write heads of the disk may experience difficulties in staying "on track," resulting in increased head positioning errors. The disk drive industry, therefore, has been challenged to overcome such head positioning problems occasioned by, among other factors, temperature cycling-induced radial disk shifts.
Various attempts have been made to address the problems caused by temperature fluctuations in a disk drive. In U.S. Pat. No. 5,334,896, the disk is clamped between the spindle motor hub flange and a disk clamp. The material of the flange is matched to the material of the disk. The disk, in this reference, is described as being supported by a seat on the spindle motor hub flange, the seat apparently extending about two third of the width of the hub flange to the outer-most edge thereof. A separate rotor yoke made of magnetic material is fitted to the hub and the yoke covers the top and the outer circumference of the stator. However, the different thermal expansion coefficients of the large yoke and of the hub, combined with the large contact surface area between the seat and the disk, may cause undesirable radial shifts and mechanical distortion in the disk as the temperature in the disk drive changes. In a further attempt to address the above-described different thermal expansion coefficients, U.S. Pat. No. 5,315,463 proposes to add a separate expansion ring and a separate polyethylene terephthalate washer between the steel hub flange and the lower-most disk of the drive. The expansion ring includes a ridge aligned with the outer diameter of the hub flange. However, because the material of the expansion ring expands at a faster rate than the rate at which the steel flange expands, the polyethylene washer disclosed therein must be interposed between the expansion ring and the steel flange, to allow the expansion ring to move freely in the radial direction with respect to the hub flange. This scheme, however, requires the addition of at least two additional and distinct parts, thereby increasing both design complexity and manufacturing costs.
What are needed, therefore, are spindle motors and disk drives that are simple to manufacture, relatively insensitive to temperature fluctuations, less costly and more effective in suppressing radial shifts or slippage than existing solutions.